Switch-mode regulators are widely used to supply power to electronic devices such as portable devices (e.g., PDA, MP3 player), computers, printers, telecommunication equipment, and other devices. Such switch-mode regulators are available in variety of configurations for producing the desired output voltage or current from a source voltage to power a load such as microprocessors of portable devices. The drive circuit is a current mirror, mirroring a fixed current, which is N times from a reference current.
FIG. 1 shows the schematic diagram of a simple current mirror circuit that has a large PFET providing a large output current of 50 mA at M0 to power a load. When CLK=0, the gate voltage of M0 is pulled high (up to VCC) to switch off the output current Io. When CLK=1, the gate of M0 is connected to the biasing voltage of M1. Because the size of M0 is of large width, there is a large current to be sunk before the voltage of the gate terminal of M0 reaches the biasing voltage of M1. Here, M1 is sized about 1/100 of M0 so that the sink current of M4 is large enough to pull down the gate of M0 to the biased voltage to match the switching frequency of the clock. If the sink current is not large enough, the gate voltage will require more time to reach the biased voltage. However, this design consumes much space and current.
FIG. 2 shows the schematic diagram of another current mirror circuit that is similar to the one shown in FIG. 1. This circuit comprises a buffer amplifier and a smaller sink transistor. The buffer amplifier limits the current discharged from the gate terminal of M0 when the CLK=1. Thus, the current sink flowing through M4 is reduced from 500 μA to 50 μA, which is ten times less than the current sink of FIG. 1. However, the buffer amplifier requires space and biasing current.
With progressing miniaturization of electronic devices and increasing speed of operation, there is an imperative need for a current circuit with less space, less power consumption and fast switching speed so that it is suitable for being employed in switching regulators.